Private branch exchange and private branch exchange control method

ABSTRACT

A private branch exchange changes the load distribution according to the use condition by a main control unit and a sub-control unit. A line card comprises a first module group normally carrying out various operations in response to an instruction of the sub-control unit, a second module group carrying out various operations in response to the sub-control unit or the main control unit, a bus arbitration circuit, and an internal module bus selecting unit having a bypass selector for bypassing the bus arbitration circuit. The main control unit predicts the load on a system at the start of the system from the information stored in the incorporated line card, and determines, considering the prediction result, whether the control is made by the sub-control unit or the main control unit, for each module of each line card.

This application is a U.S. national phase application of PCTinternational application PCT/JP02/00031.

TECHNICAL FIELD

The present invention relates to a private branch exchange and a methodof controlling the private branch exchange. More particularly, itrelates to a private branch exchange in which a main control card isequipped with a main control unit and a line card is equipped with asub-control unit, and a method of controlling this private branchexchange.

BACKGROUND ART

A variety of private branch exchanges (PBX) have been recentlycommercialized such as a PBX having a computer telephony integration(CTI) function, an ISDN-compliant PBX, and a wireless PBX.

In the foregoing circumstances, a PBX equipped with a plurality ofcentral processing units (CPU) is introduced in the market for dealingwith loads applied one after another. In this PBX, a main CPU (maincontrol unit) controls mainly applications in upper layers or carriesout an overall control. On the other hand a sub-CPU (a sub-control unit)is mounted to various line-cards and carries out mainly processing inthe terminal that needs a real-time process. The main CPU and thesub-CPU thus play their own roles independently, so that the loads aredistributed.

FIG. 4 shows a block diagram illustrating a conventional PBX, whichcomprises main control card 130, and a plurality of line cards 132 a,132 b, . . . , 132 n. Main control unit 101 of main control card 130works as a CPU of the PBX. Main memory 102 stores the programs and dataof main control unit 101. System-bus master controller 103 converts thedata into a format in accordance with its own system-bus standard,converts an address, and adjusts a timing when main control unit 101accesses respective line cards 132 a-132 n. System-bus 115 couples themain control card to respective line cards. Highway-bus 116 couples therespective line cards to each other.

In line-card 132 a, respective sections work as follows: System-busslave controller 104 receives an interface signal supplied fromsystem-bus master controller 103 of main control card 130, anddetermines whether or not the signal accesses card 132 a. Only whencontroller 104 finds the access to card 132 a, controller 104 interpretsan access mode, and converts the data received into a format of the busstandard in the line card, converts an address, and adjusts a timing.Sub-control unit 105 works as a CPU of one of line cards 132 a-132 n.Individual memory 106 stores the program and data of sub-control unit105.

First module group A 107 is directly coupled to local-bus 117, which isa bus of sub-control unit 105, and carries out a process such as dataconversion as a coder-decoder (codec). Module group B 108 a and modulegroup B 108 b produce interface signals to be supplied to first modulegroup A 107. Further the two groups carry out various processes such asa switching control or a gain control over audio data, which is dividedand placed in time slots and passes through highway bus 116 in the PBX,and a control over communications in a conference.

Internal and external lines interface 109 is an interface connectorbetween the line card and a group of telephone terminals. Intelligentblock 118 comprises local-bus interface 110, direct memory accesscontroller 111 and bi-directional memory 113.

Local-bus interface 110 harmonizes sub-control unit 105 with intelligentblock 118 so that smooth access between unit 105 and block 118 can beexecuted. Direct memory access controller 111 transfers data at a highspeed between individual memory 106 and second module groups 108 a, 108b without any help from sub-control unit 105. Bi-directional memory 113temporarily stores a command when the command is transferred betweenmain control unit 101 and sub-control unit 105

In the PBX thus structured, second module group B 108 a, which maincontrol unit 101 accesses, is clearly separated in advance from secondmodule group B 108 b which sub-control unit 105 accesses, though both ofthe groups are disposed on the same line card. Since the loads to beapplied to the main control unit and the sub-control unit can bepredicted according to a system size of the PBX, namely, the number oflines to be supported, the separation and distribution of the loads aredetermined.

Assume that main control unit 101 works on second module group B 108 bthat is under the control of sub-control unit 105, in this case maincontrol unit 101 sends a command to sub-control unit 105 viabi-directional memory 113 so that sub-control unit 105 can substitutefor main control unit 101.

As discussed above, in the conventional PBX, second module group B 108 bcan be directly controlled only by sub-control unit 105. Therefore, whensub-control unit 105 falls into an inoperable condition, an operation ofthe line card having this sub-control unit 105 cannot be guaranteed. Inthis situation, extensions governed by this line card are in trouble.

Even if first module group A 107, individual memory 106, second modulegroups B 108 a, 108 b were accessible directly from both of main controlunit 101 and sub-control unit 105, and an operation of the line cardwere guaranteed, plural stages of bus-arbitration circuits could beneeded. Whenever main control unit 101 or sub-control unit 105 accessesa module in a lower layer, and every bus arbitration is carried out,then a speed of access becomes slower, which causes the PBX to workinefficiently.

DISCLOSURE OF THE INVENTION

The present invention addresses the problems discussed above and aims toprovide a PBX that can realize a high speed access as well as guaranteean operation of a line card governed by a sub-control unit.

The PBX of the present invention comprises the following elements:

-   -   (a) a main control unit working as a CPU; and    -   (b) a plurality of line cards, and each one of the line cards        including:        -   (b-1) a sub-control unit;        -   (b-2) a first module group regularly working according to an            instruction of the sub-control unit;        -   (b-3) a second module group working according to an            instruction of the sub-control unit or the main control            unit;        -   (b-4) an internal bus arbitrator which arbitrates between            the main control unit and the sub-control unit about a right            of passing the bus; and        -   (b-5) an internal module bus selecting unit including a bus            arbitration circuit, a bypass selector for bypassing the bus            arbitration circuit, and carrying out a routing to the            second module group to be accessed from the main control            unit or the sub-control unit.

The main control unit recognizes the types and the number of theplurality of line cards on startup of the PBX, then predicts the loadapplied to the system. Based on the prediction, the main control unitdetermines which one, i.e., the main control unit or the sub-controlunit, controls which module individually of the module groups disposedon respective line cards.

Further, when the module to be accessed is controlled directly by atleast one of the main control unit or the sub-control unit, the busarbitration circuit is bypassed. When the module to be accessed iscontrolled by both of the main control unit and the sub-control unit,the inner module bus selector carries out the process of the busarbitration circuit.

A method of controlling the PBX of the present invention, including amain control unit working as a CPU and a plurality of line cards thathave their own sub-control units respectively, comprises the followingsteps:

-   -   (a) recognizing the types and the number of the plurality of        line cards accommodated in the PBX;    -   (b) predicting a load applied to the system based on the result        obtained in step (a);    -   (c) determining, based on the load applied to the system, which        module group individually disposed on the respective line cards        is controlled by which one of the main control unit or the        sub-control unit; and    -   (d) bypassing a bus arbitration process when parts of a module        group are controlled by either one of the main control unit or        the sub-control unit.

This control method can change flexibly a distribution of the load tothe main control unit and the sub-control unit based on thedetermination of performance and cost of the PBX system according to therecognized types and the number of line cards accommodated in the PBX.For instance, the sub-control unit on a line card independently controlsa module group, or receives an instruction from the main control unitand controls a module group accordingly. This method lightens the loadapplied to the main control unit, and allows the access to bypass thearbitration circuit for eliminating a useless time, thereby shorteningan access time.

When a line card is slotted in or slotted out to/from the PBX in active,the PBX can recognize correctly a type and a number of cards slotted inor out. The load can be thus distributed efficiently based on thisinformation. Further, even if a sub-control unit failed, the maincontrol unit can substitute for the failed sub-control unit and controlsthe module group under the failed sub-control unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a PBX in accordance with anexemplary embodiment of the present invention.

FIG. 2 is a block diagram detailing an internal module bus selectingunit and a second module group B in accordance with an exemplaryembodiment of the present invention.

FIG. 3 is a flowchart illustrating an operation of a PBX in accordancewith an exemplary embodiment of the present invention.

FIG. 4 is a block diagram illustrating a conventional PBX.

PREFERRED EMBODIMENT OF THE INVENTION

An exemplary embodiment of the present invention is demonstratedhereinafter with reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating a PBX in accordance with theexemplary embodiment of the present invention. The PBX comprises maincontrol card 50 and a plurality of line cards 52 a, 52 b, . . . 52 n.Main control card 50 includes main control unit 1, main memory 2, andsystem-bus master controller 3. Main control unit 1 works as a centralprocessing unit (CPU) of the PBX. Main memory 2 stores programs and dataof main control unit 1. System-bus master controller 3 converts the datainto a format of its own system-bus standard, converts an address, andadjusts a timing when main control unit 1 accesses respective line cards52 a-52 n.

Main control unit 1 fetches a program from main memory 2, reads andwrites a data, so that the main control card executes control by itself.Main control unit 1 also gives an instruction to sub-control unit 5described later via system-bus controller 3. This mechanism allows maincontrol unit 1 to control the line cards indirectly. In some cases, maincontrol unit 1 accesses directly module group A 7 and module group B 8described later, thereby controlling the line cards directly. System bus15 couples main control card 50 to respective line cards 52 a . . . 52 nin the PBX. Highway bus 16 passes audio data.

System-bus slave controller 4 provided to line card 52 a receives aninterface signal supplied from system-bus master controller 3 disposedon main control card 50. At this time, controller 4 determines whetheror not its own line card is accessed, and only when controller 4determines that the card is accessed, controller 4 interprets an accessmode. Controller 4 then converts the signal accessing its own line cardinto a format of an internal bus standard, converts its addresses, andadjusts a timing.

Sub-control unit 5 works as a CPU for one of line cards 52 a-52 n,namely, it fetches a program from individual memory 6, reads and writesa data, so that it controls module groups A 7 and B 8 by itself.Further, sub-control unit 5 receives an instruction from main controlunit 1 via bi-directional memory 13, and execute a control accordingly.Sub-control unit 5 thus lightens the load applied to main control unit1.

Individual memory 6 stores the program and the data of sub-control unit5. First module group A 7 is directly coupled to local bus 17, which isa bus of sub-control unit 5, and carries out processes such as dataconversion as a codec. Internal and external interface 9 works as aninterface connector between the line card and a group of telephoneterminals.

Intelligent block 18 includes local-bus interface 10, direct memoryaccess controller 11, bi-directional memory 13, internal bus arbitrator12, and internal module bus selecting unit 14. Local-bus interface 10harmonizes sub-control unit 5 with intelligent block 18 so that smoothaccess between sub-control unit 5 and intelligent block 18 can berealized. Direct memory access controller 11 transfers data at a highspeed between individual memory 6 and module group B described laterwithout any help from sub-control unit 5. Internal bus arbitrator 12arbitrates between an access from system bus slave controller 4 and anaccess from direct memory access controller 11. At this time,internal-bus arbitrator 12 selects an access of higher priority andoutputs the access to local-bus interface 10. Bi-directional memory 13temporarily stores a command when the command is sent between maincontrol unit 1 and sub-control unit 5.

Internal module bus selecting unit 14 has a function of bus arbitrationcircuit and a function of the bypass selector for bypassing the busarbitration circuit, and arbitrates accesses to a module, and alsocarries out a routing for the accesses.

Second module group B 8 produces interface signals to be supplied tofirst module group A 7. Further, second module group B 8 carries outvarious processes such as a switching control or a gain control of atime-slot over audio data passing through highway bus 16 in the PBX, anda control over communications in a conference.

FIG. 2 is a block diagram detailing internal module bus selecting unit14 and second module group B 8.

In internal module bus selecting unit 14, both of bypass selector 14 bfor bypassing the bus arbitration circuit and bus arbitration circuit 14a control a routing section 14 c which arbitrates between accesses to aninternal module and carries out a routing for the accesses. Bypassselector 14 b selects modules in lower priority determined in advance tobe controlled by which bus so that signals accessing those modules canbypass bus arbitration circuit 14 a.

Respective line cards have configuration memory spaces, to be morespecific, each one of system-bus slave controllers 4 of the respectiveline cards has a configuration space. System-bus master controller 3 andslave controller 4 have a configuration access function respectively, sothat the types and the number of cards slotted into the PBX arerecognized by main control unit 1 when main control unit 1 accesses theconfiguration space. Loads applied to the overall system and to therespective line cards can be thus predicted.

Each one of the configuration spaces of respective line cards storesdetailed data including a type of the card. In this embodiment, theconfiguration space stores what kind of functions the card has. As shownin FIG. 1, the number of cards can be two or more.

An operation of the PBX thus structured is demonstrated hereinafter withreference to FIG. 3. FIG. 3 is a flowchart illustrating respectiveprocesses from turning on the power supply of the PBX.

In FIG. 3, main control unit 1 fetches a program from main memory 2 atturning on the power supply of the PBX. Then control unit 1 instructssystem-bus master controller 3 to access the configuration space of afirst slot of the line card and obtain detailed data such as the typeand the number of the card supposed to be slotted into the first slotand the number of terminals supported by the line card (S1).

Not only at the startup, but also under operating conditions of the PBX,a card can be slotted-in or slotted-out, therefore the foregoingoperation can be done periodically for check purpose.

System-bus master controller 3 receives the instruction and accesses theconfiguration space of the first slot. At this time, in the case of aline card being inserted into the first slot, system-bus slavecontroller 4 reacts to this access and obtains detailed card data storedin this space. Whether or not a line card is inserted into a slot isdetermined by a response to a configuration cycle designating this card.

A response from the line card is obtained, and detailed data of the cardis read off, then a type of the card can be recognized. Further, theforegoing configuration access is also done to the second slot andrepeated up to the “n”th slot which indicates the number of the slotssupported by the PBX.

Main control unit 1 obtains the information about which slot is insertedwith what type of line card, and predicts the load applied to the entiresystem. While every slot is thus accessed, main control unit 1recognizes which slot is inserted with what type of card before the PBXsystem starts operating.

Next, based on the load data obtained in step S1, main control unit 1determines the load-distributed proportion among respective line cardsfor: module-control, namely, every module group of the respective linecards is determined to be controlled by main control unit 1 orsub-control unit 5.

Then main control unit 1 assigns a bus selection to internal module busselecting unit 14 including the bus arbitration circuit bypassingselector. This assignment is carried out such that an access speed frommain control unit 1 to the module groups to be controlled by maincontrol unit 1 as well as an access speed from sub-control unit 5 to themodule groups to be controlled by sub-control unit 5 becomes as high aspossible. The bus selection is assigned to internal module bus selectingunit 14 including the bus arbitration circuit bypassing selector (S2).

Basically module group A 7, module group B 8, and individual memory 6disposed on the line card can be accessed from both of main control unit1 and sub-control unit 5 any time and free from any restrictions.However, in this embodiment, the process of step S2 is carried out inorder to shorten the respective access times. The operation of step S2is carried out to every line card inserted in the slots.

Next, in step S3, main control unit 1 and sub-control unit 5 workaccording to the share-plan of the load determined in steps S1 and S2(S3). First, an access procedure of the following case is demonstrated:individual memory 6, or parts of module group A 7 and parts of modulegroup B 8 disposed on the line card are designated to be directlycontrolled by main control unit 1.

Firstly, main control unit 1 designates an address of a moduleregardless of the module to be accessed being disposed on the line cardor not, then carries out a regular access to the designated address.System-bus master controller 3 receives the access, and when controller3 determines the access to the line card, controller 3 produces anaccess timing in accordance with its own system-bus standard, andoutputs signals to system-bus 15.

System-bus slave controller 4 determines that the access aims at its owncard among other line-cards inserted, then controller 4 produces anaccess timing in accordance with the internal bus standard.

Internal bus arbitrator 12 receives the access, then determines whetherthe access aims at module group A 7 or individual memory 6 directlycoupled to a local bus, or module group B 8. When the access is foundaiming at module group B 8, arbitrator 12 transfers the access with theaccess timing as it is to internal module bus selecting unit 14.

Internal module bus selecting unit 14 receives the access; however,selecting unit 14 has ended a bus selection in step S2. Therefore, theaccess bypasses the bus arbitration circuit and arrives at a desiredmodule (module group B 8) without any delay.

Internal bus arbitrator 12 arbitrates a contention with direct memoryaccess controller 11 when the access is found aiming at module group A 7or individual memory 6 disposed on the line card, and arbitrator 12outputs a bus-request signal to sub-control unit 5. Then a right ofpassing through local-bus 17 is obtained before the access starts. Theaccess discussed above can be specifically carried out in the followingcases.

-   -   (1) A system has a small line capacity (a small capacity having        not many line cards), and a main control unit can control the        overall system for itself. In this case, the system can work        without a sub-control unit.    -   (2) A system has a large line capacity; however, only a small        number of line cards are slotted in, so that a main control unit        can control the overall system for itself.    -   (3) A sub-control unit fails.

When individual memory 6 or parts of module group A 7 and parts ofmodule group B 8 disposed on a line card are designated to be controlleddirectly by sub-control unit 5, the following access procedure iscarried out: Firstly, sub-control unit 5 designates an address andcarries out a regular access to the designated address. Memory 6 andmodule group A 7 can be accessed with the access timing as it is.

In the case of module group B 8, local-bus interface 10 that receivesthe access changes the access timing to the one in accordance with theinternal bus standard. Internal bus arbitrator 12 receives the accesswith the changed access timing, and transfers the access to internalmodule bus selecting unit 14 with arbitrating a contention with directmemory access controller 11. Internal module bus selecting unit 14receives the access; however, selecting unit 14 has ended a busselection in step S2. Therefore, the access bypasses the bus arbitrationcircuit and arrives at a desired module without any delay.

The access discussed above is specifically carried out in the followingcases:

-   -   (1) A system has a large line capacity, and a main control unit        cannot control the overall system for itself, so that        distributed control is needed.    -   (2) When an application in an upper layer is launched, and this        application applies a large amount of load to the CPU, or the        case where the load to the main control unit is preferably        lightened, so that the overall system desirably performs        efficiently.

As discussed above, in step S2, when parts of module groups A and B aredesignated to be under direct control of main control unit 1 or underdirect control of sub-control unit 5, the access to the module bypassesthe bus arbitration circuit. Thus the access and an operation followingthe access are carried out without any delay.

When an operation of a module under the control of sub-control unit 5 iscompulsorily changed to the one that main control unit 1 desires, thefollowing procedure is carried out: In this case, main control unit 1specifies an operation to the module under the control of sub-controlunit 5.

First, main control unit 1 accesses bi-directional memory 13 on a linecard by transmitting/receiving a command. Sub-control unit 5 monitorsbi-directional memory 13 whether memory 13 stores a new command or staysvacant by periodical poling or with an interrupt signal. Further, memory13 receives a command from main control unit 1, or addresses statusinformation thereby receiving an instruction, then sub-control unit 5operates in response to the instruction.

The foregoing access is carried out in the following case: In general,main control unit 1 changes the setting of a module under the control ofsub-control unit 5 so that an operation of the module can be changed. Inthis case, since main control unit 1 cannot directly access the module,unit 1 sends a command to sub control unit 5 temporarily via thebi-directional memory, so that sub-control unit 5 changes the setting.This type of access can be carried out when a real-time process is notrequired.

In the next step, namely step S4, whether or not a line card is slottedin or slotted out is detected. In the following cases, main control unit1 determines that a line card is slotted in or slotted out, and theprocess returns to step S1 again:

-   -   (1) Main control unit 1 receives a signal which notices that a        line card is newly slotted in.    -   (2) Main control unit 1 periodically carries out a configuration        access to the respective line cards, and recognizes based on the        presence or absence of response to the access that some of the        line cards have slotted out.

When main control unit 1 determines that a case is not applicable toeither one of the foregoing two cases, the process moves on to step S5(S4).

In step S5, an abnormal operation of sub-control unit 5 can be detectedbased on the content of a command communication via bidirectional memory13. When main control unit 1 determines that sub-control unit 5 operatesabnormally, the process moves on to step S6. When main control unit 1determines that sub-control unit 5 operates normally, the process moveson to step S3.

In step S6, all the modules detected operating abnormally are switchedto be controlled by main control unit 1. Therefore, without any specificprocess, main control unit 1 can access every module under the controlof sub-control unit 5 without restraint. Main control unit 1 thus cancontrol all the modules on the line card, so that it is assured that thesystem operates in a stable manner.

As discussed previously, the access bypasses the bus arbitrationcircuit, so that the access can be faster. Main control unit 1 thusinstructs internal module bus selecting unit 14 to carry out a busselection that entire module group B 8 should be put under the controlof main control unit 1. As a result, the access between main controlunit 1 and module group B 8 can be increasingly faster.

As discussed above, this exemplary embodiment proves that sub-controlunit 5 can independently control a module group, or receive aninstruction from main control unit 1 via bi-directional memory 13 andcontrol a module accordingly. This mechanism allows sub-control unit 5to lighten the load applied to main control unit 1.

When main control unit 1 or sub-control unit 5 accesses second modulegroup B 8, internal module bus selecting unit 14 arbitrates betweenaccesses to a module of group B 8 and carries out a routing of theaccesses. This mechanism allows main control unit 1 to substitute forsub-control unit 5 to control the module group under the control of unit5, even if sub-control unit failed.

The data about the types and the number of line cards slotted in the PBXare collected in advance. Based on the data, the load can be distributedto main control unit 1 and sub-control unit 5 and the distributionproportion can be flexibly changed according to the determination aboutthe performance and the cost of the PBX system. Further, the accessesfrom main control unit 1 or sub-control unit 5 to second module group B8 can be faster. The PBX equipped with a plurality of control units canbuild a distributed controlling system.

A distributed load processing and high reliability can be thuscompatible, and a distributed controlling system can be switched to acentral controlling system without restraint. In addition to thisadvantage, a system adequate for a line capacity can be build, so thatthe cost can be reduced.

Internal module bus selecting unit 14 can switch a bus, and recognizesdetailed data about the types and the number of all the cards slottedin. Selector 14 predicts the loads applied to main control unit 1 andsub-control unit 5 based on the line capacity supported by the systemand the data about the types and the number of the line cards. Theprediction prompts selecting unit 14 to prepare a bypass selector thatallows an access to bypass a bus arbitration circuit. The module groupsthus can be switched without restriction to be controlled by maincontrol unit 1, or sub-control unit 5, or controlled by both of units 1and 5. When the bus arbitration circuit is not needed, the access canbypass the arbitration circuit for shortening the access time.

Internal bus arbitrator 12 arbitrates between an access from system-busslave controller 4 and an access from direct-memory access controller11. The arbitration prompts arbitrator 12 to select the access of higherpriority and output it to local-bus interface 10.

The PBX discussed above has the following modes:

-   -   (1) a mode that allows both of main control unit 1 and        sub-control unit 5 to access every module of second module group        B 8;    -   (2) a mode that switches some modules out of second module group        B 8 to be accessible only from main control unit 1 to the ones        accessible only from sub-control unit 5, or vice versa.

The availability of the foregoing two modes allows an access to themodules accessible only from main control unit 1 and the modulesaccessible only from sub-control unit 5 to bypass the bus arbitrationcircuit, so that an access can be faster.

A conventional PBX includes some line cards that are not equipped with asub-control unit for cost reduction purpose. This structureadvantageously distributes the control of the respective line cardsbetween the main control unit and the sub-control unit according to thetypes of the line cards; however, a problem exists in an access time.The PBX of the present invention, on the other hand, can realize afaster access.

System-bus slave controller 4 includes a configuration space therein.System-bus master controller 3 and system-bus slave controller 4 have aconfiguration access function respectively. This structure allows thePBX to recognize the types and the number of the card slotted in becausemain control unit 1 can access the configuration space.

Based on the information about the configuration space, the load appliedto the overall system and the load to the respective line cards can bepredicted. Therefore, at turning on the PBX or whenever a line card isslotted in or out, the access to the configuration space allows the PBXto recognize correctly the types and the number of cards newly slottedin or slotted out. Based on the information obtained from this access,the load can be distributed more efficiently.

INDUSTRIAL APPLICABILITY

The PBX of the present invention recognizes the types and the number ofline cards slotted therein, thereby adjusting flexibly a loaddistribution between a main control unit and a sub-control unit. Thisadjustment lightens the load applied to the main control unit, allows anaccess to bypass a bus arbitration circuit when an arbitration is notneeded, thereby shortening the access time. When a line-card is newlyslotted in or slotted out, the PBX can recognize the types and thenumber of the cards newly slotted in or slotted out. Based on thosedata, the load can be distributed efficiently. In the PBX of the presentinvention, even if the sub-control unit failed, the main control unitcan substitute for the sub-control unit to control the module groupunder the control of the sub-control unit. The PBX of the presentinvention can thus guarantee the operation of the line cards and realizean access at a high speed.

1. A private branch exchange (PBX) comprising: (a) a main control unitfor working as a central processing unit (CPU); and (b) a plurality ofline cards, and each one of said line cards including: (b-1) asub-control unit; (b-2) a first module group for regularly workingresponsive to an instruction from said sub-control unit; (b-3) a secondmodule group for working responsive to an instruction from one of saidmain control unit and said sub-control unit; (b-4) an internal busarbitrator for arbitrating between said main control unit and saidsub-control unit about a bus right of passing through a bus; and (b-5)an internal module bus selecting unit having a bus arbitration circuitand a bypass selector for bypassing said bus arbitration circuit,wherein when one of said main control unit and said sub-control unitaccesses said second module group, said internal module bus selectingunit carries out a routing of the access, wherein said main control unitrecognizes types and numbers of said line cards at startup of the PBXfor predicting a load applied to a system, and the prediction promptssaid main control unit to determine which one of said main control unitand said sub-control unit controls which individual module of the firstand the second module groups on each one of said line cards, whereinwhen a module to be accessed is directly controlled by at least one ofsaid main control unit and said sub-control unit, the access bypassessaid bus arbitration circuit, and wherein when the module to be accessedis controlled by both of said main control unit and said sub-controlunit, said internal module bus selecting unit carries out a processusing said bus arbitration circuit.
 2. The PBX as defined in claim 1,wherein the PBX detects whether or not a line card out of said pluralityof line cards is slotted in or slotted out while the PBX is in active,and wherein when the PBX detects one of the slot-in and slot-out of theline card, the PBX predicts a load applied to the system.
 3. The PBX asdefined in claim 1, wherein the PBX detects whether or not saidsub-control unit operates abnormally, and an abnormality prompts saidmain control unit to substitute for said sub-control unit to control amodule of a line card from which the abnormality is detected.
 4. The PBXas defined in claim 1, wherein each one of said line cards furtherincludes (b-6) a bi-directional memory temporarily stores a command whenthe command is transferred between said main control unit and saidsub-control unit; wherein said main control unit obtains the bus rightusing said internal bus arbitrator before accessing directly said firstmodule group; and wherein when said main control unit accessesindirectly said first module group, said main control unit transfers aninstruction to said sub-control unit via said bidirectional memory, sothat said sub-control unit executes the instruction from said maincontrol unit for controlling an operation of a module to be controlled.5. A method of controlling a private branch exchange (PBX) that includesa plurality of line cards, each one of the cards being equipped with asub-control unit, and a main control unit working as a centralprocessing unit (CPU), the method comprising the steps of: (a)recognizing types and numbers of said respective line cards included inthe PBX; (b) predicting a load to be applied to a system based on aresult obtained in step (a); (c) determining, based on the load appliedto the system, which one of said main control unit and said sub-controlunit controls which individual module group provided to each one of saidline cards; and (d) bypassing a bus arbitration process when a part ofmodules of the module group are controlled by one of said main controlunit and said sub control unit.
 6. The method of controlling a PBX asdefined in claim 5 further comprising the step of: (e) executing the busarbitration process when a module to be accessed is controlled by bothof said main control unit and said sub-control unit.
 7. The method ofcontrolling a PBX as defined in claim 5 further comprising the steps of:(e) detecting whether or not a line card is slotted out or slotted inwhile the PBX is in active; and (f) executing again steps (a) and (b)when a line card is slotted out or slotted in while the PBX is inactive.
 8. The method of controlling a PBX as defined in claim 5 furthercomprising the steps of: (e) detecting whether or not said sub-controlunit operates abnormally; and (f) controlling modules of a line card,which includes said sub-control unit detected an abnormality, with saidmain control unit instead of said sub-control unit.
 9. A private branchexchange (PBX) comprising: (a) a main control unit for working as acentral processing unit (CPU); (b) a plurality of line cards, and eachone of said line cards including: (b-1) a sub-control unit for sharing aload; (b-2) a first module group for executing processes includingencoder-decoder (codec) conversion of an audio signal; (b-3) a secondmodule group for receiving an instruction from one of said main controlunit and said sub-control unit, and executing at least one of processesincluding a generation of an interface signal to said first modulegroup, a switching control of a time slot over the audio signal, and acontrol over telecommunication in a conference; (b-4) an internal busarbitrator for obtaining a bus right of passing through a bus from saidsub-control unit and transferring the bus right to said main controlunit when said main control unit accesses said first module group; and(b-5) an internal module bus selecting unit having a bus arbitrationcircuit and a bypass selector for allowing an access to bypass said busarbitration circuit, and carrying out a routing of an access, whereinsaid main control unit recognizes types and numbers of said line cardsat startup of the PBX for predicting a load applied to a system, and theprediction prompts said main control unit to determine which one of saidmain control unit and said sub-control unit controls each module of saidrespective module groups disposed on each one of said line cards,wherein when a module to be accessed is directly controlled by at leastone of said main control unit and said sub-control unit, the accessbypasses said bus arbitration circuit, and wherein when the module to beaccessed is controlled by both of said main control unit and saidsub-control unit, said internal module bus selecting unit executes aprocess of said bus arbitration circuit.
 10. The PBX as defined in claim9, wherein said internal module bus selecting unit includes a busswitching function which has two modes; (i) every module of said secondmodule group can be accessed by both of said main control unit and saidsub-control unit, (ii) parts of modules of said second module group canbe accessed only by said main control unit, which can be switched to,parts of modules of said second module group can be accessed only bysaid sub-control unit, and vice versa.
 11. The PBX as defined in claim 9further comprising: a system-bus master controller, wherein each one ofsaid plurality of line cards further including a system-bus slavecontroller for adjusting a timing between a system-bus and each one ofbuses of said plurality of line cards, wherein said system-bus slavecontroller has a configuration space, and both of said system-bus mastercontroller and said system-bus slave controller have a configurationaccess function respectively.
 12. The PBX as defined in any one of claim9 through claim 11, wherein said internal module bus selecting unitcarries out a routing to a module to be accessed of said second modulegroup when one of said main control unit and said sub-control unitaccesses the module.
 13. The PBX as defined in claim 9, wherein saidinternal module bus selecting unit arbitrates between two buses on anaccess contention.
 14. The PBX as defined in claim 13, wherein the PBXdetects whether or not said sub-control unit operates abnormally, anddetects whether or not any line card of said plurality of line cards isslotted out or slotted in while the PBX is in active, and predicts aload to be applied to the system when the slotted-out or slotted-in isdetected, and wherein when said sub-control unit is detected anabnormality, said main control unit controls modules of a line card ofsaid plurality of line cards, from which line card an abnormality isdetected.
 15. A private branch exchange (PBX) comprising: (a) a maincontrol unit; and (b) a plurality of line cards, and each one of saidline cards including a sub-control unit and a plurality of modules,wherein said main control unit recognizes types and numbers of saidplurality of line cards at startup of the PBX, and based on therecognition, said main control unit determines each module of theplurality of modules is controlled by which one of said main controlunit and said sub-control unit.
 16. The PBX as defined in claim 15further comprising: (c) a bus selector including a bus arbitrationcircuit and carrying out a routing to a module to be accessed, whereinwhen a module to be accessed is directly controlled by at least one ofsaid main control unit and said sub-control unit, the access bypassesthe bus arbitration circuit.
 17. The PBX as defined in claim 15, whereinthe PBX detects whether or not any line card of said plurality of linecards is slotted out or slotted in while the PBX is in active, and adetection of the slotted-out or slotted-in prompts said main controlunit to recognize types and numbers of said plurality of line cards, andbased on the recognition, said main control unit determines each moduleof said plurality of modules is controlled by which one of said maincontrol unit and said sub-control unit.
 18. The PBX of claim 15, whereinthe PBX detects whether or not said sub-control unit operatesabnormally, and when an abnormality is detected in operation of saidsub-control unit, a module included into at least one line card of saidplurality of line cards, which includes said sub-control unit detectedthe abnormality, is controlled by said main control unit.